1. Field of the Invention
The present invention relates to a monolithic bidirectional power switch controllable on and off.
2. Discussion of the Related Art
The term “power switch” is here used to designate a switch capable of switching at voltages greater than 100 volts and/or at currents greater than 1 ampere. A power switch is for example intended to switch the power supply of a load connected to the mains. A power switch is generally formed vertically in a semiconductor substrate, that is, the current between the main electrodes of the switch flows between the upper and lower surfaces of the semiconductor substrate. This differentiates power switches from the components of an integrated circuit, which are generally horizontal, that is, the main electrodes of these components are located on a same surface of the semiconductor substrate and the current flows horizontally between these main electrodes.
An on and off controllable device, such as a vertical MOS transistor, may be used as a power switch. However, a vertical MOS transistor is unidirectional and it is necessary to associate it with a diode bridge if an A.C. current is desired to be conducted. This association has the disadvantage of having a non-negligible voltage drop in the on state. This is also true for a bipolar transistor.
A bidirectional component such as a triac, which has the advantage of having a very small voltage drop in the on state, may also be used. However, this component has the disadvantage of not being controllable to be turned off, unless a complex control circuit is associated thereto.
FIG. 1 shows a vertical bipolar NPN-type power transistor. At the surface of a lightly-doped N-type (N−) semiconductor substrate 1 is formed a heavily-doped N-type (N+) region 3 in a P-type well 5. Substrate 1 and region 3 respectively form the collector and the emitter of the transistor. Well 5 forms the transistor base. On the lower surface of substrate 1 is formed a heavily-doped collector contact N-type layer 7. Respective emitter, base, and collector metallizations E, B, and C are formed on regions 3, 5, and 7.
Substrate 1 is relatively thick, which provides a significant off-state breakdown voltage. Indeed, intrinsic silicon can withstand approximately 10 V per micrometer of silicon. Thus, a vertical structure comprising a substrate 1 having a thickness on the order of 60 μm may withstand, in the off state, a voltage on the order of 600 V.
If a power transistor with a bidirectional operation is desired to be obtained, it can be envisaged to make the structure of FIG. 1 symmetrical, for example, as illustrated in FIG. 2.
The structure of FIG. 2 comprises a stack of a first thick lightly-doped N-type semiconductor layer 11, of a thin lightly-doped P-type layer 13, and of a second thick lightly-doped N-type layer 15. Layer 13 forms the base of the symmetrical transistor and layers 11 and 15 form its emitter/collector regions. Heavily-doped N-type emitter/collector contact regions 17 and 19 are formed, respectively, on the upper surface of layer 15 and on the lower surface of layer 11. A heavily-doped P-type (P+) well 21 crosses semiconductor layers 15 and/or 11 and contacts base region 13. Respective emitter/collector and base metallizations A1, A2, and B are respectively formed on regions 17, 19, and 21.
The symmetrical structure of FIG. 2, however, cannot operate properly. Indeed, emitter/collector regions 15 and 11 are lightly N-type doped, which makes them efficient as collector regions (high breakdown voltage) but inappropriate as emitter regions, since they do not enable a proper injection of carriers in the on state. Thus, in both operating directions, the structure of FIG. 2 has a low gain. To improve the on-state carrier injection, the doping of regions 15 and 11 should be increased, but this would decrease the breakdown voltage. Further, it is difficult to form a thin deeply buried base region 13 with current techniques, and the forming of a well 21 poses problems and may have side effects.
A bidirectional switch, controllable to be turned on and off, is thus needed.